The present invention is directed toward a method of automatically testing a plurality of repeaters connected along a pulse code modulation transmission route.
A transmission line of a pulse code modulation system (hereinafter referred to as PCM system) includes a number of repeaters provided at regular intervals, to perform regeneration of signal pulses. A known method of testing such repeaters, and of locating a faulty repeater, is based upon providing a filter at the output of each repeater, with each filter being of band-pass type, for selecting an individual frequency from a frequency component which can be included in a pulse train applied to the repeater. The frequency which is selected by a filter is referred to as the supervisory frequency, and an individual supervisory frequency is thus allocated to each of the repeaters. The outputs from these filters are connected in common to a return transmission path. In order to locate the position of a faulty or inoperative repeater, a pulse train containing a frequency component identical to the supervisory frequency of a first one of the repeaters is applied from a terminal of the transmission line. If a supervisory signal is sent back along the return transmission path, then this indicates that the corresponding repeater is operative. A pulse train containing a frequency component identical to that of the supervisory frequency of the next succeeding repeater is then applied from the terminal to the transmission line. This process is repeated until a repeater is found for which no supervisory signal is sent back along the return transmission path, or for which the level of the supervisory signal returned is below a predetermined level. This indicates that the corresponding repeater is faulty. Since the supervisory frequency is known, the location of the faulty repeater is thereby determined.
By a slight modification of this method, the operating margin of each repeater along a transmission line can be measured. This is accomplished by successively increasing the density of a pulse train in the signal applied to the transmission line. Polarity alternation of the pulse train serves to produce the supervisory signal component of the applied signal, and increasing the density thereof should produce a corresponding increase in the amplitude of the supervisory signal returned by a repeater. If, for a known change in the density of the pulse train, the amplitude of the resultant increase in the supervisory signal is below a predetermined level, then this indicates that the supervisory signal component of the applied signal is causing the repeater concerned to make a large number of errors, i.e. misjudgements of the signal pulses applied thereto. The point at which this occurs serves to indicate the degree to which each repeater is operating to the design specification, i.e. the degree of disturbance in the input signal pulses up to which the repeater will continue to successfully regenerate these pulses.
Such prior art methods are performed manually. In other words, an operator selects each of the supervisory signal frequencies in turn and applies a corresponding test signal to the transmission line, to successively test the various repeaters. In the case of a line having a large number of repeaters, this is a time-consuming process. The method of the present invention overcomes this problem by testing each repeater, and measuring its operating margin, completely automatically,